This invention relates to on-board diagnostics and, more particularly, to an on-board diagnostic method and system for diesel vehicles that determines the current hydrocarbon (HC) conversion based on the exotherm generated by a reductant injection strategy and a theoretical exotherm assuming complete conversion of the injected hydrocarbon and provides an indication if the current HC tailpipe emittance exceeds a target threshold.
It is desirable to add reductant to the lean environment of diesel exhaust to reduce NOx emissions. When injecting reductant into the engine exhaust, the burning of hydrocarbons across the catalyst releases heat, which increases the post-catalyst temperature. This resulting rise in temperature is directly related to catalyst efficiency, the concentration of HC in the exhaust, and is a time dependent function of engine exhaust flow. As shown in FIG. 1, as the catalyst ages catalyst efficiency degrades, and HC light-off is delayed to higher catalyst temperatures. For a new vehicle, the catalyst is fresh and the secondary fuel injection system is fully functional, the catalyst efficiency is expected to be high, and the fuel injection quantity correct. Under these conditions, the degree of temperature rise, DT, should be at its maximum possible value. As the vehicle mileage increases and the catalyst ages, HC conversion efficiency decreases accordingly. This reduction in efficiency will reduce the degree of temperature rise, DT, from the fresh value. Monitoring the degree of temperature rise, DT, provides a means to estimate catalyst efficiency, which in turn can be used to infer whether required emission levels are being met. While temperature sensors have been used in the past to monitor catalyst activity (e.g., U.S. Pat. Nos. 5,201,802 and 5,706,652), the prior art approaches have not used an injected reductant quantity specifically for and sized to enhance catalyst performance monitoring.
In accordance with the present invention, a method is proposed for evaluating catalyst efficiency by monitoring catalyst exotherm level. More particularly, a diesel diagnostic strategy for evaluating catalyst efficiency is proposed that includes injection of a sufficient amount of HC into the engine exhaust stream to maintain a constant concentration of reductant into the catalyst under variable engine exhaust flow conditions thereby permitting the calculation of a theoretical exotherm. The ratio of the actual to theoretical exotherm is calculated to provide an indication of the actual HC conversion by the catalyst. The actual conversion is compared to data for a catalyst aged to threshold emittance levels to determine whether the vehicle is meeting emission regulations.